An employee has various duties to his employer. By way of general observation only, these include - whether specifically mentioned in the employment contract or not:
1. Not working against the employer's interests while still an employee.
2. Not using customer lists, or any information obtained in course of employment, for your own personal gain.
If you were in any way diverting work or business opportunities from your old employer to your new company while still an employee, then I think you definitely have a difficulty. The situation would be worse if you were a director with your old employer, but even so, there is still a definite problem if you were acting as outlined .
Consult a solicitor.